This invention relates to a high pressure fuel injection unit for an engine and more particularly to an improved accumulator type injector nozzle for engines.
A popular and very effective type of injection nozzle for injecting fuel to an internal combustion engine is the so called "accumulator type". With this type of injection nozzle, there is provided an accumulator chamber in which fuel is stored under pressure and which can communicate with the engine through a discharge port. An injection valve is mounted within the accumulator chamber and controls the opening and closing of the discharge port. There is also provided a control chamber that receives fuel under pressure from the accumulator chamber and the injection valve has a portion that is supported within this control chamber. As a result, the control chamber pressure acts on the injection valve and assists in maintaining it in a closed position. A control valve selectively relieves the pressure in the control chamber. When this occurs, the pressure of the fuel in the accumulator chamber will cause the injection valve to open and fuel to be discharged. With this type of system, it is also the practice to employ a biasing spring that acts on the injector valve so as to urge it to its closed position in cooperation with the fuel pressure in the control chamber.
Although this type of injection nozzle is very effective, it is difficult at times to provide good running at low engine speeds such as idle. The reason for this is that at low pressures under low speed operation the amount of fuel discharged will increase significantly for a given increase in fuel pressure. This may be understood best by reference to FIG. 1 which is a graphical view showing the fuel pressure in the accumulator chamber in relation to the amount of fuel discharged during four varying running conditions, indicated by the curves a, b, c and d. The curves a, b, c and d indicate respectively increasing engine speeds with the curve a being idle speed and the curve d being wide open throttle, maximum engine speed. It can be seen that the curve a is quite sharp. Hence a small difference in fuel pressure in the accumulator chamber will make a substantial difference in the amount of fuel injected.
It is very difficult to control adequately the pressure of the fuel in the accumulator chamber at low engine speeds. This is because the fuel pressure changes with a number of variables primarily at temperature, which effects the viscosity.
In order to reduce these fluctuations in amount of fuel discharge at low engine speeds in response to pressure variations, it has been proposed to increase the biasing preload of the spring that cooperates with the pressure in the control chamber to hold the injection valve in its closed position. Although an increased spring preload will lessen the slope of the curves shown in FIG. 1, this gives rise to another problem. That is, at high engine speeds when the actual time duration of opening of the injection nozzle is quite short, it may be impossible to inject sufficient fuel so as to achieve maximum performance. Therefore, the previously proposed systems have not been effective in solving the aforenoted problem while at the same time maintaining maximum engine output.
It is, therefore, a principal object of this invention to provide an improved accumulator type injection nozzle.
It is a further object of this invention to provide an improved accumulator type injection nozzle that will provide good fuel control at low speeds even with pressure variations.
It is a further object of this invention to provide an improved accumulator type injection nozzle wherein the sensitivity of fuel discharge at low pressures is reduced without adversely effecting the amount of fuel which can be discharged at high pressures.
It is a further object of this invention to provide an improved arrangement for operating an accumulator type injection nozzle so as to cause smaller changes in fuel discharge with pressure changes under low pressure conditions than at high pressure conditions.